Recently, in wire harnesses to be mounted in cars, better water resistance is desired for electric wires with a terminal including an insulated electric wire and a metal terminal provided at the end portion thereof. Electric wires with a terminal having water resistance include a waterproofing resin portion that covers a waterproof region at the end portion. Hereinafter, electric wires with a terminal including a waterproofing resin portion are referred to as “waterproof electric wires with a terminal”.
The waterproofing resin portion is made of a synthetic resin material and is formed by insert molding using, as an insert portion, a protected region including a region between an insulating coating at the end portion of the insulated electric wire and a portion where the metal terminal is connected to a core wire so as to cover the protected region.
Moreover, in a process for producing the waterproof electric wire with a terminal, before the insert molding of the waterproofing resin portion is performed, an adhesive is applied all over the circumference of the surface of the insulating coating at the end portion of the insulated electric wire. This adhesive becomes an adhesive layer that bonds the insulating coating and the waterproofing resin portion while closing a gap between the insulating coating and the waterproofing resin portion. This adhesive layer also serves as a water stopping portion that prevents the infiltration of water.
In the waterproof electric wire with a terminal, the coefficient of thermal expansion of the insulating coating is different from the coefficient of thermal expansion of the waterproofing resin portion, and therefore, the adhesive layer needs to have elasticity capable of coping with the difference in the thermal expansion between the insulating coating and the waterproofing resin portion. Generally, an adhesive of silicone based resin or rubber based resin having water resistance and elasticity is used as a material for an adhesive serving as a water stopping agent.
However, when the insulated electric wire includes an insulating coating of polyolefin based resin that is an olefin based resin, use of the adhesive layer of silicone based resin or rubber based resin results in a poor adhesiveness (water stopping performance) between the insulating coating and the waterproofing resin portion. Therefore, water easily infiltrates into the portion where the core wire and the metal terminal are connected to each other, which causes corrosion and contact failures in the core wire and the portion to which the metal terminal is connected.
On the other hand, JP 2006-123458A shows that in the process for producing the waterproof electric wire with a terminal, an adhesiveness between the insulating coating of olefin based resin and the adhesive is enhanced by applying corona discharge treatment or plasma discharge treatment to the insulating coating of the insulated electric wire.
However, if corona discharge treatment or plasma discharge treatment is performed in the process for producing the electric wire with a terminal, production man-hours increase and the introduction of expensive facilities is needed.